A painting by Mark Rothko which was vandalised at Tate Modern has gone back on display after 18 months of restoration work.

Wlodzimierz Umaniec defaced the 1958 painting Black On Maroon and was jailed for two years for the crime in December 2012.

The vandal, who co-founded the artistic movement "yellowism", stepped over a barrier in the gallery and daubed his name and the words "12 a potential piece of yellowism" before fleeing.

Rothko donated Black On Maroon, one of his acclaimed Seagram murals, to the Tate in 1970.

It was vandalised with graffiti ink in October 2012.

Tate said the damage will always remain under the surface of the work but it had now been conserved to "displayable condition".

Tate director Sir Nicholas Serota said the painting was one of the best-known and most-loved in its collection.

He said: "I am delighted that everyone can once again come to Tate Modern and see Rothko's magnificent Black on Maroon.

"Looking after its collection, Tate has a conservation team that is one of the best in the world. Their expertise, rigour, patient work and respect for the painting has enabled us to return it to public view, as envisaged by Mark Rothko."

Sir Nicholas said that when he heard about the attack on the painting, which has been valued at £50 million, it felt "appalling" and "sickening".

At a press conference to announce the work going back on display, he said: "To be in a position where you have the responsibility to care for these works, to hear that one of these has been damaged, it's an appalling feeling, a sickening feeling, partly because we had no idea whether it was going to be possible to restore it."

He added that the painting was "given by Rothko to everyone in Britain, everyone in the world, and it was in our care when it happened. It was really ghastly news".

"To see the extent of that damage later that day, one had no idea whether it would be possible to restore it or not."

He added that the damage had been done to the whole group of Seagram murals because Black On Maroon was part of the series, designed to be shown together in a meditative environment at Tate Modern.

"The damage was not just to one painting, but to nine. It was a really ghastly feeling," he said.

He said the conservation work had surpassed expectations, adding: "There was a hope that we would be able to do something but the result is much more successful than we could have hoped for."

He said security had been reviewed in the wake of the incident but they did not want to turn the gallery into "Fort Knox or a prison".

"This is a gallery, not a prison. We want people to enjoy the paintings in the way in which Rothko meant them to be enjoyed," he said.

Rothko painted the murals in the late 1950s, in a commission for the fashionable Four Seasons restaurant on Park Avenue, New York.

But the maroon, dark red and black murals were darker in mood than his previous work and so he withdrew from the commission.

He finally presented the series, which he saw as objects of contemplation, to Tate in 1970, expressing his deep affection for England and for British artists, especially JMW Turner.

The conservation on the painting was long and difficult, partly because Rothko often used unusual materials, including eggs, oils, pigments, colourants, resins, and glues, to create his works.

Rachel Barker, paintings conservator at Tate, said the process was so slow that on some days she was only able to remove less than an inch of ink at a time.

The indelible graffiti ink had penetrated several layers, in some cases soaking through to the back of the canvas.

Tate had to find a chemical solvent which could remove the ink while limiting damage to the original paint.

A team spent nine months analysing microscopic samples of layers on the painting and whittling down hundreds of potential solvents until they found the right one - a blend of benzyl alcohol and ethyl lactate - before even attempting to restore the work.

Special test canvases were used to assess solvents and cleaning methods, while the Rothko family also donated a canvas for testing - one which the artist had primed with maroon paint at the time of the Seagram commission in the 1950s.

A further nine months was spent working on Black On Maroon, removing the majority of the surface ink before reversible conservation-grade materials were used to restore the painting's surface.

The work was carried out in consultation with the Rothko family.

The painter's son Christopher Rothko said: "The Rothko family has been repeatedly impressed by the thoroughness and dedication of the Tate conservation team.

"They have realised the only satisfactory resolution to a terrible situation: the work is once again on display for the public as our father intended."